There is increasing societal sensitivity to environmental issues, including restrained use of organic solvents, volatile organic compounds (VOCs) and other additives such as coalescent agents due to health concerns. One area in which the foregoing has become important is architectural coatings, especially aqueous latex paints and water borne coating compositions. As a result, aqueous latex paints and water borne coating compositions are gaining popularity.
In practice, however, it is a challenge to minimize the use of organic solvents, VOCs and additives without diminution of the performance of the coatings. Coalescence is a process whereby polymer particles in an aqueous latex or dispersion come into contact with one another during film-forming process and polymer chains diffuse across boundaries of latex/dispersion particles to yield continuous films with good bonding of the polymer particles. There is a balance between achieving the desired hardness of a resulting dried film from an aqueous paint composition and having the proper coalescence.
A method of improving the properties of films formed by water-borne compositions is to include polymer latex particles that are capable of being crosslinked. In one example, diacetone acrylamide (DAAM) is included in the latex particles as moieties. Crosslinking agents within the aqueous composition but not part of the latex particles can be used to crosslink the polymer latex particles when the aqueous composition is applied to a substrate under ambient conditions. Adipic dihydrazide (ADH) is one of these crosslinking agents that have been used for aqueous latex polymers. However, the conventional approach of using diacetone acrylamide (DAAM) as part of the latex and adipic dihydrazide (ADH) as free floating agents in the aqueous composition sometimes allows crosslinking reactions between the DAAM and ADH to take place during storage, i.e., while the latex particles are in the aqueous phase. This may diminish shelf stability and reduce intra particle crosslinking under ambient drying conditions. Other conventional two-component crosslinking approaches have similar stability issues as well as VOC and odor concerns.
U.S. Pat. No. 3,838,104 to Hayashi et al. discloses an oil and water-repellent copolymer by copolymerizing diacetone acrylamide (DAAM), diacetonemethacrylamide (DAMAM) or a lower alkylol derivative thereof and a fluoroalkyl monomer. Hayashi does not disclose an aqueous latex composition that may crosslink when applied to a substrate under ambient conditions. CN 101348541 to Li discloses a crosslinking acrylate composite emulsion. The functional monomers include diacetone acrylamide (DAAM) and adipic dihydrazide (ADH). U.S. Pat. No. 3,451,480 to Zeh discloses a copolymer of acrylamide and diacetone acrylamide (DAAM) as a friction reducer in brines used in oil well fracturing. Up to 0.006% of N,N′-methylene bisacrylamide may be included as a cross-linking agent. WO 03/068880 A1 describes a latent cross-linking thickener composition including diacetone acrylamide (DAAM) and adipic dihydrazide (ADH).
Lu et al. (Zhongguo Jiaonianji, 15 (1), 2006, pp. 17-20) describes a blend of self-crosslinking acrylate latexes. Each of the latexes is copolymerized from diacetone acrylamide (DAAM), acrylamide (AM) or N-methylacrylamide, among other monomers. However, a blend of DAAM containing copolymer with an AM containing copolymer shows a lack of water resistance. U.S. Pat. No. 3,497,467 to Coleman discloses an aqueous polymer latexes having diacetone acrylamide (DAAM) monomer as the main component. The DAAM dominant polymer can be a copolymer with a vinyl or another acrylic monomer. WO 02/087734 A1 discloses hydrophobic membranes modified with a surface coating that includes a cross-linked ter-polymer. The monomers include diacetone acrylamide (DAAM), N,N′-methylenebisacrylamide and N,N-dimethylacrylamide.
All patents and publications discussed herein are incorporated by reference herein in their entirety.
There remains an unmet need for an aqueous latex composition that is stable during storage while still being capable of crosslinking when applied on a substrate without additional water borne crosslinking agents such as ADH during or after drying under ambient conditions. Crosslinking after film formation would improve film strength, block and water resistances and bring other benefits to the water borne coatings all while achieving proper film formation.